In liquid crystal display screens, the backlight sources have great influence on the display of the whole picture, and direct illumination-type backlight sources with multiple divisions can match an image and then a better picture display effect can be obtained as each division can separately modulate the light intensity, for example, usually a high contrast can be obtained and large power consumption can be reduced. Theoretically speaking, the more divisions a backlight region has, the higher the contrast will be and the larger the reduction of power consumption will be.
Currently a liquid crystal television has about 100-500 backlight sources. For the whole circuit system, in terms of the traditional architecture of an AC to DC converter plus a DC to DC converter plus a constant current control chip, due to its increased number of voltage conversions, the efficiency of the whole system is lowered greatly. Also, the unwieldy circuit system renders the whole circuit hardware very complex, the television occupies much space, and then the appearance of the television is greatly affected.
To solve the above problems, inventors put forward an architecture of AC to DC plus a constant current control chip. This architecture can omit the voltage conversion link of DC to DC converter, and thus omit many hardware devices and also improves system efficiency. However, due to the architecture of an AC to DC converter and its cost restriction, the cost of multiple AC to DC converts is high, and they occupy a large space, thus the present invention will use a method that one AC to DC converter corresponds to all LED lamp strings. While, how to ensure a stable working state of all the LED lamp strings is also a problem that should be solved urgently.
Due to the influence of processes, the forward voltages of current LED lamps at a rated current would float within a certain range, and taking a certain lamp as an example, its forward break-over voltage is within the range of 2.8-3.6V and its center voltage is 3.0V. Thus, the voltage difference between the LED lamp strings may have great deviation theoretically. Taking a string with four lamps as an example, in the state of the rated current, the maximum voltage and the minimum voltage of the LED lamp string are 14.4V and 11.2V, respectively, and the center voltage is 12V.
In the situation of using the architecture of the AC to DC converter, suppose it should be ensured that theoretically all lamp strings should work at a rated current, the output voltage of the AC to DC converter should be enhanced to 14.4V (which should be higher actually considering the voltage drop at a constant current source). In this situation, the constant current control chip at the LED lamp string with a center voltage of 12V will bear a differential pressure loss voltage of 2.4V. This voltage will be completely transformed into thermal loss, the heat will dissipate into the system, and the heat loss is ΔQ=I (the rated current of the LED)×2.4V. According to the theory of statistics, if there are enough LED lamp strings, the forward voltages of the LED lamp strings at the rated current should conform to Normal distribution. The voltages of the majority of the LED lamp strings are about 12V. Thus, if the system outputs a voltage of 14.4V, the thermal loss of the system is considerable. Taking a backlight source having 1,000 lamp strings each of which has 4 lamps and having a current of 20 A as an example, the center value of the thermal loss is 48 W while the actual power consumption of the lamps are 240 W. Thus, this will greatly lower the efficiency of the system and make the whole system generate huge heat, which results in rather high risk.
To solve the above technical problem and improve the reliability of the system, currently a manner of completely self-adaptive voltage is used to obtain a suitable voltage. That is, in one LED lamp string backlight system, self-adaptive voltage compensation is conducted to the LED lamp string of the current backlight system, so that all the LED lamp strings of the whole LED lamp string backlight system work at a rated current. However, if there are too many LED lamp strings, according to the statistical principle of the Normal distribution, in one system, the possibility will increase sharply that the voltage of a certain LED lamp string is far higher than 12V. Then the reliability of the self-adaptive voltage manner would decrease greatly.
In addition, if the self-adaptive voltage control manner is not used while the manner of fixed voltage output is used, it would be very difficult to select the output voltage. The lamps made by different manufactures could hardly meet compatibility.
Therefore, there is a need of a backlight drive voltage control technology which can solve the above technical problems.